Conventional selective call receivers operate to receive radio frequency signals using a radio frequency amplifier that is typically tuned for optimal performance over a narrow operating frequency band. In order to maintain certain objectives such as battery life and profitability, a manufacturer will typically impose power and cost constraints that result in performance trade-offs.
As an example of a performance trade-off necessary in low power receiver design, a designer, given the goal of optimizing a radio frequency amplifier for low power consumption and good sensitivity over a relatively narrow frequency band, might choose to sacrifice dynamic range or intermodulation distortion characteristics. This choice would possibly lead to an amplifier having poor distortion characteristics at high input signal levels. Given that most conventional low power radio frequency amplifiers are of either a common emitter neutralized or a cascode topology, each of limited dynamic range, the choice of improving sensitivity by several decibels over a similar improvement in distortion characteristics would probably be a wise one. The preceding choice can be justified since the aforementioned topologies are well suited for amplifying a desired signal in environments where interfering signal levels are substantially below a desired signal level. However, in an environment where in addition to the desired signal, undesired narrow or broadband interference is impressed upon the amplifier, the desired signal will not be adequately amplified, thus degrading the receiver's sensitivity.
The degradation in sensitivity previously mentioned is due to the amplifier responding to the undesired signals falling within its operating bandwidth. More particularly, any number of signals falling within the operating bandwidth of an amplifier of finite dynamic range will cause the amplifier to generate an amplified response. The amplified response will correspond substantially in proportion with each of the input signals, but only to the point where the amplifier has sufficient power to respond in a linear fashion to said signals. When the amplifier becomes overloaded with respect to the desired signal, the result is a distorted amplified output signal. Essentially, a portion of the total energy available for amplification of the desired signal is "consumed" by the amplifier when it responds to amplify any interfering signals. In an ideal amplifier, this problem never occurs, but when constrained as discussed before, a designer will be eternally confronted with the choice between designing an amplifier having high gain, low noise figure, and poor high level distortion performance; or an amplifier having low gain, medium noise figure, and improved high level distortion performance.
Thus, what is needed is an apparatus, that in conjunction with a radio frequency receiver and amplifier, yields a receiving system having a relatively constant receiver sensitivity over widely varying interfering signal conditions. Moreover, the apparatus must operate in a power conserving mode while maintaining an amplified signal gain appropriate for the impressed signal conditions. As a result of controlling the signal gain, the distortion characteristics of the radio frequency receiver and amplifier are improved.